1. Technical Field
The present disclosure relates to diagnosing and treating cognitive deficits involving attention, sequential processing, reading, navigation, and speed of processing, deficits that occur naturally with age, and for treating reading disorders, such as dyslexia. More particularly, the present disclosure relates to methods and apparatus for measuring contrast sensitivity for motion discrimination at both high and low levels of cortical processing and to improving contrast sensitivity for motion discrimination.
2. Description of Related Art
When an image falls on the retina, it is processed within the retina to some extent. Retinal ganglion cells send signals out of the eye to a relay nucleus in the thalamus of the brain. Cells of the thalamus, in turn, send signals to the visual cortex for further processing. There are two major types of retinal ganglion cells, which respectively contact two classes of cells in the relay nucleus of the thalamus:                Parvocellular neurons and magnocellular neurons. Parvocellular neurons have small receptive fields and respond to visual tasks requiring a high degree of acuity.        Magnocellular neurons, which are about one-tenth as numerous as parvocellular neurons, have large receptive fields and respond to visual tasks requiring a high degree of movement sensitivity. They have coarse acuity and high contrast sensitivity.        
In view of the above, the human visual system may be divided into two visual streams. The first is the dorsal stream, predominantly composed of magnocellular neurons, which detects movement. This dorsal stream has a high sensitivity to low contrast (for example, below 10%), to low luminance, to movement, and has low resolution. The second is the ventral stream, composed of both parvocellular and magnocellular neurons, which detects the color, shape, and texture of patterns. This second, or ventral, stream has low contrast sensitivity and high resolution. The ventral stream is most sensitive to contrasts above about 10%.
The parvocellular and magnocellular neurons, either alone or in combination, provide the information used by many different visual cortical pathways (or “streams”), which are specialized for performing different perceptual tasks. One such specialized pathway is a visual cortical area called Medial Temporal, or “MT”, which is central to analyzing the direction of motion. Most of the signals that drive neurons in area MT derive from neurons in layer 4b of the primary visual cortex, V1, which, in turn, are primarily activated by input from the magnocellular cells (in humans, V1 is the only cortical area that receives signals from the retina via the thalamic relay nucleus.) Direction selectivity is a fundamental characteristic of magnocellular neurons in the dorsal stream, and is mediated by cells in both layer 4b and 6 of the primary visual cortex, V1, and in the MT cortex. In direction discrimination tasks, magnocellular neurons in the dorsal stream signal in advance of the linked parvocellular neurons in the ventral stream, which are sensitive to patterns. The timing and direction of visual events in the direction-selectivity network is signaled by magnocellular neurons, activated at pattern onset and offset. Detailed pattern information used to identify each word is signaled by parvocellular neurons, which provide the background frame of reference for judging the direction of movement.
If magnocellular neurons are a substrate of reading, then one would expect physiological and psychophysical plasticity in the neural channel's sensitivity to take place at the same time as functional changes in the cortical organization used for reading. Reading involves the coordination of saccadic eye movements, requiring the integration of information from the temporal and frontal lobes, and pattern recognition, requiring the integration of information from the occipital, temporal and parietal lobes. The temporal lobe shows peak synaptogenesis, i.e. developing and pruning synaptic contacts, at 6 to 10 years which corresponds with the time the child is learning to read. Moreover, both temporal and frontal cortical areas continue to develop into young adulthood. Experience refines the output of cortical circuits by introducing patterned activity that fine-tunes the strength of neuronal connections within and among cortical columns. Even in adulthood, brain plasticity results from a continuing process of experience-dependent synaptogenesis. Perhaps, during a time of peak developmental plasticity, as when the child is learning to read, the cortical neuronal connections are especially plastic. Direction discrimination is still developing in young children.
Certain aspects of the magnocellular networks, such as direction discrimination and the ability to detect brief patterns are still developing in 5 to 9 year old children as compared to normal adults, so children aged 5 to 9 years, both normal and dyslexic, benefit from training to improve motion discrimination. Moreover, there is increasing psychophysical, physiological, and anatomical evidence that dyslexics have anomalies in their magnocellular networks manifested by (1) higher contrast thresholds to detect brief patterns, (2) an impaired ability to discriminate both the direction and the speed of moving patterns, and (3) unstable binocular control and depth localization when compared to age-matched normals. The lack of synchronization in timing between magnocellular and parvocellular activations, caused by sluggish magnocellular neurons, may be what has been disrupted in dyslexia, shown schematically in FIG. 2, resulting in temporal and spatial sequencing deficits that slow reading speeds and processing speed. The dyslexic reader's more sluggish, magnocellular neurons may cause a deficit in attentional focus, preventing the linked parvocellular neurons from isolating and sequentially processing the relevant information needed when reading.
In older adults, timing deficits manifest themselves as an impaired ability to pay attention, navigate, perform figure/ground discrimination, and process information sequentially and quickly. Processing speed deficits may underlie the cognitive decline found in older adults; the prevalence of these deficits increases with age. There is functional Magnetic Resonance Imaging (fMRI) evidence that, just like is found for dyslexics, cortical areas V1 and MT in older adults show less activation than do those areas in younger adults. In addition, older adults have a decreased sensitivity to radial motion, i.e. optic flow, analyzed in cortical area MST, that is directly related to impaired navigational skills. Moreover, behavioral data show that adults over 50 have decreased contrast sensitivity to stationary and moving sine wave gratings at low spatial frequencies under all viewing conditions. These contrast sensitivity losses may be due to age-related changes in magnocellular pathways.
Recent fMRI studies indicate that older adults exhibit more dorsal prefrontal activation when performing tasks than their younger counterparts as a result of needing to expend more processing effort. This is consistent with a degradation of magnocellular pathway timing. Reduced information processing speed, resulting from sluggish magnocellular neurons, may explain problems in memory encoding and retrieval because this mental slowing can lead to superficial processing and inefficient strategies where elaboration is required.
For older adults, training regimens employing motion detection and discrimination combined with object recognition improved processing speed, functional field of view, and performance in driving and other mobility and navigation tasks. These training regimens present high contrast patterns that are not as effective in improving cognitive deficits, as found when using low contrast grayscale patterns for direction discrimination training. Moreover, none of these training regimens improved the contrast sensitivity for motion discrimination, which is directly related to the amount reading speed, or processing speed, improved. Since the output of neurons in the visual cortex is directly related to pattern contrast, and stimulus contrast modulates cortical functional connectivity, high contrast patterns are not able to train the motion or dorsal stream, and instead train the ventral stream. Other methods to improve processing speed present patterns with such rapidity that they are not of sufficient duration and of low enough contrast to train magnocellular neurons at the anterior portion of the dorsal stream. Cognitive therapies, like remembering a sequence of letters and/or digits, are designed to activate the ventral and not the dorsal stream. Conventional wisdom in the art, e.g. the Mayo clinic, teaches that either doing nothing but monitoring the patient or providing medications that have not yet been shown to be effective in treating mild cognitive impairments are used to manage age-related cognitive impairments. Therefore, none of these interventions may improve cognitive impairments as efficiently or effectively as done by training direction discrimination between patterns that optimally activate magnocellular neurons.
A natural assumption in the art is that reading relies on the high-resolution acuity system, which may be evaluated by measuring visual acuity using the familiar Snellen index (20/20, 20/40, and so on as known in the art). Conventional wisdom in the art teaches that dyslexia, which may be defined as a reading deficit in a child of normal intelligence and an adult-level acuity (i.e. 20/20), is a difficulty in decoding or encoding words on a page that are readily seen. Most reading therapies concentrate on improving phonological awareness instead of improving processing in the motion-sensitive dorsal stream, as occurs with direction discrimination training, the invention being disclosed. Vision-based reading therapies, integrated with phonological training, work with high contrast patterns that are aimed at improving processing in the ventral stream (identifying the letters in a word and the words on a page) and not the dorsal stream.
Motion sensitive (magnocellular) sensitive neurons must be activated to remediate the underlying direction discrimination deficit in the dorsal stream. The posterior portion of the dorsal stream is trained, when using the original invention, to be more sensitive and respond more quickly to discriminating the direction of movement. Others claim that the posterior portion of the cortex is not able to be trained to improve reading, and these assumptions have now been proven to be false, since direction discrimination training using patterns that activate magnocellular neurons significantly improved reading fluency. Moreover, differences between children with reading problems (e.g. those who are dyslexic) and children with normal reading skills were revealed only by tests focusing on the cortical movement system. Tests focusing on the pattern system, such as assessing visual acuity or word recognition using long duration patterns, were unable to discriminate between children with normal reading skills and children with reading problems. Furthermore, no other vision therapy has been found to improve phonological deficits, suggesting that improving the timing in the brain may be the foundation needed to improve a wide range of reading deficits (both visual and phonological) and cognitive deficits, including attention, figure/ground discrimination, sequential processing, speed of processing, navigation, and visual memory.
The motion contrast sensitivity training that has thus far been provided, however, may fail to: 1) incorporate larger numbers of differing neural channels at each level of cognitive processing, and 2) train motion discrimination at higher levels of cognitive processing in the dorsal stream, sharpening the attention gateway, so figure/ground discrimination, sequential processing, visual memory, and navigation can be done effortlessly. Tasks at higher levels in the dorsal stream, most notably in the dorsal lateral prefrontal cortex where sequential processing, executive control of attention, and visual working memory are computed, are trained by remembering a sequence of patterns. There may be no intervention available that uses patterns that maximally activate magnocellular neurons to train a person to remember a sequence of patterns. Therefore, there may be no interventions to improve cognitive processing in the anterior portion of the dorsal stream.